Process for developing electrostatic image with liquid developer



Oct. 6, 1959 K. A. METCALFE ET AL PROCESS FOR DEVELOPING ELECTROSTATIC IMAGE WITH LIQUID DEVELOPER Filed Dec. 19, 1956 TOCONDUCTOR Z FACED PAPER United States Patent PROCESS FOR DEVELOPING ELECTROSTATIC IMAGE WITH LIQUID DEVELOPER Kenneth Archibald Metcalfe, Graymore, South Australia, and Robert John Wright, South Payneham, South Australia, Australia, assignors to The Commonwealth of Australia, care of the Secretary of the Department of Supply, Melbourne, Australia Application December 19, 1956, Serial No. 629,363 Claims priority, application Australia December 29, 1955 20 Claims. (Cl. 117-37) This invention relates to improvements in and relating to electro-photography and electro-radiography, also referred to as xerography and xeroradiography, and in particular it relates to certain improvements whereby the image can be produced and developed in a simpler and more effective manner and with a higher degree of definition and permanence.

In electro-photography it is customary to use a plate or paper having a layer of photo-conductive material, such as amorphous selenium, which in the dark will accept and hold an electrostatic charge, this plate being then exposed in any manner to vary the charge thereon according to the pattern of exposure to light or other influence and the plate is subsequently developed by applying. thereto a powder having a charge, so that when the powder is applied to the plate it will be accepted by the plate in conformity with the charge on the various sections of the plate, this then producing an image in the powder.

One of the difficulties with this type of development has been to be able to distribute the powder finely and in uniform consistency because obviously if the powder is unevenly applied, the image will similarly be unevenly built up and will not be a true picture of the original exposure.

Another difliculty is to fix the powder on the plate in a permanent manner because obviously in industrial application of electro-photography it is essential that a permanent image will result, this permanence being at the present time usually obtained by including a resin which, after the dust has been deposited on the plate, can be fixed by subjecting the resin to heat.

1 All of the present methods, however, are open to the disadvantage that, by whatever method the powder is applied, it is extremely difficult to obtain uniform coverage of the plate to ensure a build-up of the powder in exact proportion to the image left by exposure of the charged plate, and to overcome some of these ditficulties it has been proposed for instance to blow the powder on to a plate, or to roll it on to the plate by means of an appropriately charged roller or by minute beads which carry it, or to use a liquid which is sprayed in the general direction of the plate but the droplets of a varied by applying which dry as they reach the plate so that they adhere to the plate only in those areas where the so formed powder has the correct polarity in relation to the charge on the plate.

The objects of the present invention are to improve the image and to produce it in a better manner, this having been achieved by what may be termed a wet development of the image, the invention being directed to the production of an electrostatic image on a suitable surface which is then developed on that surface, or transferred to another surface, and developed by wetting the surface with a carrier liquid having an electrical resistance suificiently high to prevent destruction of the electrostatic image and carrying dispersed in it a pigment which has a charge of the correct polarity, or has added p 2,907,674 1C6 Patented Oct. 6, 1959 to it a control agent to modify the charge, to act in conjunction with the electrostatic image on the surface to develop the image. A fixing agent may also be dispersed in the said carrier liquid to render the pigment fast when the carrier liquid is evaporated or removed, and the fixing agent may, by suitable selection, also be the control agent.

It will at once be apparent that Where the pigment material is carried in a neutral high resistance liquid which will not destroy the charge On the plate, at least during the development period, the whole of the image will be uniformly subjected to the liquid in which the colour material is evenly and generally relatively thinly dispersed and this will result in rapid deposition of the colour material in a density varying in proportion to the charge held by the various parts of the image, and as the suspending liquid is preferably a highly volatile one,-

evaporation of this liquid from the developed image will rapidly leave the image in a dry condition so that, if a fixing agent is added to the carrier liquid, or if the developer has that characteristic, the pigment is left adhering rigidly to the surface which has been developed. The high resistance insulating liquid also allows the particles carried by it to have and retain the necessary electrical characteristic, which characteristic can however be a control agent to the suspended matter.

By this means an extremely fine image results because, unlike the dry process which gives relatively coarse results due to particle size limitations or agglomeration effect, a liquid developer is found to deposit in a form resembling colloidal or a near colloidal form or possibly extending to colloidal form. Extreme enlargements of images so produced have been found possible without adverse grain effect.

It will be unnecessary hereto to elaborate on these advantages, excepting to mention the obvious difi'iculty which existed heretofore when it Was necessary to carry a dry powder of extremely fine particle size in air, necessitating a carrying agent such as the beads, Whereas such particles can with ease be carried in a suitable liquid and can be deposited therefrom in the desired uniform manner.

To give some idea of how the process may be carried out, some detailed descriptions now follow, but it is to v be remembered that these are by way of example only and the process is not limited to these.

In'these examples certain substances will be referred to under their well known trademarks, these being,

Rhodene L6/100 a linseed oil modified alkyd resin of medium oil length,

Pentarol 20 a phenol modified penta-erythritol ester of rosin,

Fusis A a high boiling point, high aromatic solvent, and

Monolite Yellow and Monolite Red both fast organic pigments of the colour mentioned.

As an example, a base material such as paper or other suitable surface is coated with zinc oxide and an alkyd resin suspended in a drier and thinner so that after application of the solution the paper can be dried.

For instance, the proportions of a typical coating mix: ture'would comprise: Zinc oxide, made by the indirect process, predominantly non-fluorescent, with an index of refraction of 2.0 such as the paint pigment known as Durham.

D.C. source, the fiat metal plate being connected to the positive electrode. Typically the paper is subjected to the corona discharge for a period of 10 seconds when the point is held 2 inches above the paper. For the point there may be substituted a wire or group of wires, or a sharp knife edge of say steel, in which case the distance may be reduced between the wire or knife and the paper and the voltage reduced to give similar electrical field strength in terms of volts per cm., or alternatively a charge may be applied by positioning the paper between condenser plates or the like and this paper is then exposed by subjecting it to the action of light in a camera or the like or by impressing on it or in any way which will cause an image to be produced on the electrostatically charged surface of the paper which can subsequently be developed.

It has been found also that the electrostatic charging may in some cases be effected after exposure of the paper to the image-producing means.

Development then follows by simply applying a liquid developer to the paper, this being as said applied by submerging the paper beneath the liquid or by pouring it on or spreading it with a roller or other applicator or electrode, the developer liquid consisting of a liquid insulating medium which carries the pigmented particles and as an example of this the liquid may be a cyclohexane which has a very high resistance and a low dielectric constant, in which liquid is dispersed carbon black or any other suitable pigment and a control agent to modify or give to the pigment the required electrical charge. For instance if the pigment is to have a positive charge an alkyd resin control agent and fixer may be included in the liquid, whereas if a negative charge is required linseed oil may be used as the control agent and fixer.

An example of a positively charged developer paste which selectively deposits on a negative electrostatic image is as follows:

Grams Pentarol 20 l5 Rhodene L6/ 100 15 Fusis A 25 Phthalocyanine blue 150 Grams Boiled linseed oil 25 Lead "chromate r 75 Drier, lead naphthenate 0.5

The ingredients are milled together for eight hours in a ball mill.

In both cases, the paste or mixture is dispersed in a liquid to form a dispersion of high volume resistivity. An example of the method of dispersion is as follows: stir a small portion of the past into in? I liq such as n-pentane or carbon tetrachloride, and strain to remove portions of paste that have not completely dispersed. For fine grained developers filter before use through filter paper and use filtrate as a developer. The concentration of the dispersion may be varied at will, a typical concentration of a preferred dispersion being a suspension of 0.5 gram developer paste in mils of liquid.

One of the advantages of a developer of this nature is that as the pigment particles are contained in a high resistance liquid and are therefore insulated, all of the particles will take the characteristic of the control agent such as the alkyd resin or the linseed oil, which may also be the fixing agent, this then ensuring that deposition of the material forming the image will take place in a highly uniform manner in direct proportion to the positive or negative charge existing on the paper due to the application of the image thereto. 1

It will of course be realised that amixture for instance of cyclo-hexane and carbon black paste, in the proportions 0.5 gram of carbon black paste to 100 mils of cyclohexane or generally in the range 0.05 gram to 5 grams of carbon black paste to 106 mills of cyclo-hexane gives a very quick drying compound which when it has once been applied to the surface will quickly achieve permanence in that the cyclo-hexane will be evaporated and will leave the carbon black in position in a manner just as permanent as ink printing or the like, the invention therefore resulting in a method which will allow very rapid development of an electrostatic image and moreover will ensure that a high degree of permanence is at once present which requires no further treatment to maintain its permanency.

It has been found that the use of wet developers of this type and of the aforementioned concentrations re-. sults in complete development in about 15 seconds ,when applied by immersion of the paper or plate in the developer in a metal or glass or enameled metal dish.

If the developer is applied as the plate or paper is passed between conducting rollers or relatively conducting rollers which are thoroughly wetted with the developer, the developing time is reduced to less than 1 second, preferably better specified in feet per second such as 200 linear feet per second. It is emphasised that with the developer concentrations stated previously the development time is about 15 seconds in dish development but if the concentrations are very low then longer development such as 30 seconds may be necessary to bring up the full colour saturation required.

Where a conducting surface, such as a metal plateor an applicator roller or the like is used during development the movement of the pigment particles to the image is accelerated, and such a surface may itself serve also to pick up and convey away these pigment particles which are rejected by the electrostatic image, this ensuring a cleaner image.

The drawing designated Fig. 1 shows diagrammatically how development with a liquid developer takes place, the floor of the tray being designated 1, the paper carrying the surface charge 2, the liquid 3 and a roller 4.

It is to be noted that when the paper 2 is submerged in the liquid 3 a flow of the pigment particles, indicated as dots, takes place somewhat in the direction of the arrows, the particles of course moving freely under the known laws governing particles suspended in insulating liquids, but because of their polarity being moved to the charged areas 5 of the paper 2 to be attached to the areas, the movement of the'particlesdenuding the liquid at the areas 6 to leave that surface clear, the action of the roller assisting rapid development by acting as an electrode to intensify movement of the particles and also acting to bring particles into proximity with the charged surface of the paper 2 and to receive particles (area 7) where the particles are denuded from the areas 6.

"The use 'of wet developersof this type results in the production of an image of extremely fine grain size thus giving a higher degree of definition than is available by other means either by electro-photography with dry development or by silver halide photography at equivalent speeds. For the selenium plate a resolving powerof not less than 1000 lines per millimeter is readily achieved.

It will of course also be apparent that it would be possible to utilise two diiferent pigments or two difierent coloured pigments in the liquid and to so select the material thereof that the two have opposite electrical char acteristics and therefore will be deposited on different parts of the exposed surface to developat once the positive and negative sections thereof in different colours. An example of a developer containing two pigments of oppositepolarity, selected to develop at once the negative and positive sections thereof in different colours is as follows:

Paste A, negatively charged: Grams Boiled linseed oil 75 Cadmium selenide (red) 75 Driers, lead napthenate 0.5

Paste B, positively charged:

Monolite yellow G.N.S 75 Cyclo-hexane 100 Add 0.5 of each of these pastes to 100 mils of cyclohexane and disperse thoroughly.

It has also been found possible to use the process in superimposing images of different colour and in this regard it may be mentioned that by exposing a paper a number of times and each time developing with a different colour, a coloured image may be built up which in effect allows colour photography to be attained in a very simple and effective manner.

As an example of the way in which coloured developers may be made, reference may be made to the previously given composition for a positively charged developer containing phthalocyanine blue: For the phthalocyanine blue there may be substituted a number of other coloured pigments suchas Monolite red and Monolite yellow or certain well known inorganic pigments such as cadium red or chrome yellow. It is emphasised that these coloured developers are readily mixed to give mixed colours such as purple, orange, brown or the like, or by the addition of carbon black to give shades and by the addition of white pigments such as zinc oxide or the like, to give tints, and by the addition of black or white pigments to give shade-tints. It will be appreciated that the possibility of colour development by the super-position of colours depends on the transparencies of the pigments used, as in colour printing, as well in this case as on the electrical resistivity of the dry pigment after deposition on the paper or plate. However, there are no major difliculties in superimposing the common pigments one on top of the other to form a colour picture. It will be further appreciated that it is possible to control the amount of each individual colour deposited by exposure or development alone or together i.e. colour correction is possible in processing.

Further, it is possible to develop in three or more colours in a novel way. Development in one colour, say red, is arrested before the electrical charges on the paper or plate are satisfied, whereupon the paper or plate is immersed in another developer of, say, blue, and portion of the image becomes blue; this procedure is again followed with, say yellow. The result is a colour picture which however does not necessarily render the correct colours for the subject.

It will also be clear that the invention can be utilised with the already well known selenium plate, the image from this being simply developed with the liquid or the image being transferred to another treated surface which may then be developed in this way.

When applying the invention to electro-radiography the surface to be subjected to the X-rays is painted with a heavy metal oxide such as lead oxide which is capable of absorbing greater quantities of radiations, the surface or Grams Rhodene L9/50 resin Zinc oxide Yellow litharge (massicot, PbO) 20 Cobalt naphthenate 0.5 Manganese naphthenate 0.5 Toluene 100 These materials are milled together in a ball mill for eight hours.

The material may be applied to a paper or metal or other surface by brushing, spraying, dipping, electro-coating or other technique.

Another material used to paint such surfaces is as.

follows:

. Grams Rhodene 1.9/50 resin 50 Yellow litharge (massicot) PbO 200 Cobalt naphthenate -4. 0.25 Manganese naphthenate 0.25

Thinners, toluene as required.

It is emphasised that a basis for selecting the metal oxides or other materials for X-ray purposes is to use the one containing an element which has an absorption edgev in the band of wave lengths which it is particularly desired to use for X-raying purposes. For example, in the above formula, the active substance lead oxide contains lead which has an absorption edge appropriate to the X-ray kilovoltage 70 kvp.

Here again of course it is possible to wrap a paper or the like on to the object and to develop this and'also when using ordinary radiological examinations such as for clinical purposes a much more efiiective method is provided in that the paper is simply treated in the manner described and can then be immediately developed to produce the pattern of the X-ray, the final record being permanent so that it may at once be stored without any fear of deterioration.

We are aware that it has been proposed heretofore to atomise liquid by means of a stream of air and to charge the atomised droplets by passing them through a field and to then deposit the droplets, probably after they have reached a dry stage, on to the pattern of an electrostatic image.

We are aware also that printing inks have been used which contain some of the ingredients of the liquid developer, but it must be mentioned that printing inks themselves would not be useable as a liquid developer because they would have a consistency such that mechanical deposition would take place in preference to electrostatic selection.

' According to our invention the pigment must be suspended in a suificient body of carrier liquid, of an electrical resistance so high as to prevent destruction of the electrostatic image charge, to allow the pigment particles to move freely in the liquid so that the phenomena of particle charge can serve as the deposition control, the particles moving freely in the liquid but having a surface charge according to their nature, or as modified by the modifying agents referred to, so that these particles themselves are selectively drawn to the image areas where pigment is. required but are repelled at those areas Where pigmentshould not settle, the invention therefore differing from the prior art in that particles which carry their own inherent charge, whether modified or not, can move freely in an insulating liquid, which liquid itself is incontact with the image in a mass sufficient to allow the free movement of the particles for the selective action.

We claim:

1. A process for developing an electrostatic image on a base material comprising mixing a pigment with alkyd resin, dispersing the resulting mixture in a liquid to form a dispersion of high volume resistivity, the alkyd resin establishing an electrical surface charge adapted for enabling the mixture to be deposited selectively on said electrostatic image, and applying the dispersion to a surface carrying a negatively charged electrostatic image for thereby forming a visible image on said surface.

2. A process as claimed in claim 1, is a cyclohexane. I

3. A process as claimed in claim 1, wherein the liquid is carbon tetrachloride. I p

4. A process as claimed in claim 1, wherein the liquid is n-pentane.

5. A process as claimed in claim 1, wherein the pigment and resin are mixed with toluene.

- 6. A process as claimed in claim 1, wherein the mixture consists of particles having an average size of 0.5 micron.

7. A process as claimed in claim 1, wherein the mixture is prepared in the form of a paste and about 0.5 to 5.0 grams of the paste are dispersed in about 100 mils of said liquid.

8. A process as claimed in claim 1, comprising mixing a plurality of different pigments with said resin.

9. A process as claimed in claim 1, comprising fixing the visible image on said surface by means of a fixing agent.

10. A process as claimed in claim 1, wherein the pigment and resin are mixed by milling.

' 11. A process for developing an electrostaticimage on a base material comprising mixing a pigment with linseed oil, dispersing the resulting mixture in a liquid to form a dispersion of high volume resistivity, the linseed oil establishing an electrical surface charge adapted for enabling the mixture to be deposited selectively on said herein the liquid electrostatic image, and applying thedispersion to a surface carrying a positively charged electrostatic image for thereby forming, a visible image on said surface.

12. A process as claimed in claim 11, wherein said linseed oil is boiled linseed oil.

13. A process as claimed in claim 11, wherein the liquid is a cyclohexane. i 4 14. A process as claimed in claim. 11, wherein the liquid is carbon tetrachloride.

15. A process as claimed in claim 11, wherein the liquid is n-pentane.

16. A process as claimed in claim 11, wherein the linseed oil and pigment are mixed with a drier.

17. A process as claimed in claim 11, wherein about 0.5 to 5.0 grams of the mixture are added to about mils of said liquid.

18. A process as claimed in claim 11, comprising ernploying said linseed oil for fixing the visible image on said surface.

19. A process as claimed in claim 11, wherein the pigment and linseed oil are mixed by milling.

20. A process as claimed in claim 11, wherein the pig ment consists of substance normally having opposite polarities when dispersed in said liquid.

References Cited in the file of this patent UNITED STATES PATENTS 2,049,507 McBurney et a1. Aug. 4, 1936 2,221,776 Carlson Nov. 19, 1940 2,297,691 Carlson Oct. 6, 1942 2,392,376 Goepfert Jan. 8, 1946 2,761,416 Carlson Sept. 4, 1956 2,784,109 Walkup Mar. 5, 1957 FOREIGN PATENTS 1,112,180 France Nov. 9, 1955 OTHER REFERENCES Metcalfe: Liquid Developers for Xerography, Journal of Scientific Instruments, vol. 32, February 1955, pp. 74 and 75.

Healey: Physical Properties of Printing Inks, Paint Manufacture, March 1943, XIII, 3, pp. 48-52, 106-28.

Unkefer et al.: Driers for Printing Inks, American Ink Maker, October 1950, pp. 57-67, 106-28. 

1. A PROCESS FOR DEVEOLPING AN ELECTROSTATIC IMAGE ON A BASE MATERIAL COMPRISING MIXING A PIGMENT WITH ALKYD RESIN, DISPERSING THE RESULTING MIXTURE IN A LIQUID TO FORM A DISPERSION OF HIGH VOLUME RESISTIVELY, THE ALKYD RESIN ESTABLISHING AN ELECTRICAL SURFACE CHARGE ADAPTED FOR ENABLING THE MIXTURE TO BE DEPOSITED SELECTIVELY ON SAID ELECTROSTATIC IMAGE, AND APPLYING THE DISPERSION TO A SURFACE CARRYING A NEGATIVELY CHARGED ELECTROSTATIC IMAGE FOR THEREBY FORMNG A VISIBLE IMAGE ON SAID SURFACE. 